Surface treatments for substrates, e.g., metal, are conventional to inhibit corrosion and improve adhesion of subsequent coatings. One such surface treatment is phosphate conversion coating that phosphatizes the surface by producing a non-conductive phosphate crystalline/amorphous layer on the surface that insulates the metal from subsequently applied coatings and provides a topograpy having chemical functionality for reacting with the coating.
Representative of the phosphate conversion coating processes are an iron phosphate coating process and a zinc phosphate coating process that use a bath of iron phosphate or zinc phosphate, respectively. Both processes require multiple steps including meticulous cleaning pretreatment of the surface, dipping in the bath and rinsing followed by application of the coating. If the cleaning pretreatment is not properly performed, the surface will be defective in that it will have less corrosion protection, and hence will be more likely to corrode, than a nondefective surface. The iron phosphate coating process requires close chemical control of the bath to achieve reproducibility and satisfactory results. The zinc phosphate coating composition is more complex and expensive than the iron phosphate coating process and requires additional chemical additives. Some of these additives are environmentally hazardous. The removal and disposal of sludge produced in the bath is a serious environmental problem. If the sludge contaminates the surface to be coated, then the coated metal surface will be defective.
In the multi-step processes, phosphate coatings are quite porous and begin to degenerate shortly after application if not quickly recoated with a suitable primer. In less than four days the unprotected iron phosphate film will deteriorate to a point where there is little corrosion protection.
The multi-step processes can also include a chromic acid rinse to seal the phosphatized metal surface prior to applying the coating. The chromic acid is usually hexavalent chromium which is highly toxic and therefore undesirable.
The multi-step processes also utilize a large amount of volatile organic compounds (VOCs) that are hazardous to the environment.
U.S. Pat. No. 1,995,954 to Albrecht discloses a coating composition containing phosphoric acid and rust-inhibiting derivatives thereof as rust-resisting agents and a paint. The Albrecht composition lacks an additive package having complexes and a cross-linking agent in the paint, both of which are disclosed herein.
U.S. Pat. No. 4,032,678 to Perfetti et al. discloses a thermosetting organic coated sheet metal product having a partially cured coating which bonds to the metallic substrate (C1 L50-52). Perfetti does not disclose a single coat self-phosphatizing paint because the organic coating is subsequently treated with a protective and/or decorative coating applied thereon as the top or finish coat (C2, L64-C3, L2).
U.S. Pat. No. 4,508,765 to Ring et al. discloses synthetic resins and coating compositions for coating metal substrates, particularly food and beverage cans. The resin includes the reaction product of a water insoluble phenolic resin or a water insoluble amino resin with an epoxy resin and a phosphoric acid (C1, L46-50). The reaction of the phenolic or amino resin with the epoxy and acid occurs prior to the application of the composition on a substrate.
Lin et al., Chemistry of a Single-Step Phosohate/Paint System, Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 discloses a single-step phosphate/paint system that utilizes phosphoric acid, a resin and a cross-linking agent. The phosphoric acid results in the paint system being unstable as the acid can react with the other paint components or catalyze the reaction of the components. The stability and compatibility of the Lin paint are limited and only clear, unpigmented paints have been produced. The Lin paint requires a large amount of solvent, which increases the VOCs, that is miscible with both the phosphoric acid and the other components. If the Lin paint utilizes a resin and cross-linking agent combination other than polyester - melamine enamel or a pigment, it becomes useless in a short period of time because of the reactions, e.g., the self-condensation of the cross-linking agent, the co-condensation reaction of the resin with the cross-linking agent and aggregation of the pigment due to the highly acidic (pH&lt;2.0) condition, of the components prior to application to the surface.
There is a desire for a single coat, in situ self-phosphating paint that is easy to use, does not require meticulous cleaning of the surface, eliminates the need for separate phosphate conversion and subsequent coating steps, eliminates the bath, reduces VOCs, is stable and can be pigmented. Preferably, the paint will protect plastic and wood surfaces in addition to metal surfaces. The present additive package can be used to make a paint that satisfies at least some of these desires.